I. Intro

II. On the Science of Yoga

III.The Actual Science

IV. The Vagus Nerve

V. Sympathetic and Parasympathetic Systems

VI. Polyvagal Theory

VII. Vagal Tone and Behavior

VIII. Putting it All Together

IX. References




Yoga is downright amazing. We move, we flow, we stretch, and we get to listen to an ungodly amount of unsubstantiated claims by our instructors. Its fucking wonderful.

“Let’s twist out the toxins!”

“Inversions are great because they reverse the blood flow.”

There are many commonplace statements being made about what we’re doing as we move through our practice that many of us either simply accept because it is being expounded upon by someone we trust, or else outright ignore because we simply don’t care.

The end result in either case is that misinformation gets spread. There are a myriad of fitness myths that pervade our day-to-day existence, and yoga is no different: there are things we hear and eventually accept for no better reason than we don’t want to research these claims ourselves. We attribute a certain amount of credibility to the person with whom we choose to lead us through a practice and, because of this, will readily accept most, if not all, that this person says.

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On the Science of Yoga

Thankfully, there’s a whole movement in place to make yoga a more genuine practice. In “Yoga Needs to Cut the Bullshit,” Janine Zeitlin suggests that much of modern yoga discussion remains “stuck in New-Ageville.” She goes on to say that “claims ensuring swift physical and mental transformation detract from compelling headway on how yoga can actually help.”2 And I agree – when you come to expect specific outcomes within a relatively short amount of time, your entire practice suffers. It’s highly unlikely that your cancer will be cured in one headstand. It’s highly unlikely you’ll lose thirty pounds in a month. It’s highly unlikely that you’ll be free and clear of depression in a small number of sessions.4

When expectations like these are set, it’s easy to become discouraged with the practice because these expectations are often not being met. Does this mean that yoga cannot accomplish these things? … Not necessarily. However, what yoga can or cannot accomplish is not so easily explained or understood, and our ignorance on the matter is not alleviated in wanting to believe everything we’re told.

When we’re informed, we’re better equipped to evaluate claims that are being made in a practice many of us cherish. Rather than discuss various claims individually, this article will lightly touch upon current research in the fields of biomechanics; anatomy and physiology; and phylogeny to provide a jumping off point for deeper study and, hopefully, provide a better understanding of why yoga may or may not live up to some of the claims regarding emotional and behavioral modification.

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The Actual Science

So how does yoga actually help? Well, its actually all in your head.

Strength and flexibility have far more to do with neural systems than just muscle. It isn’t about how far you can stretch or how much you can lift – it would be better said that it’s how far your brain will allow you to stretch and how much your brain will allow you to lift. The nervous system sends lines of communication throughout the entire body, with tendrils reaching into every one of our muscles and tendons and organs. The sensory information being sent to the brain from these nerve endings provide information that the brain may interpret as safe or harmful. When pertaining to flexibility, these messages may say “we can go farther,” or “no, too fast! We need to stop!” In the case of strength, these messages may be interpreted as “the weight is fine,” or “put the weight down.” The brain interprets these messages accordingly and sends the appropriate message back to the muscles and tendons, which will say either “great, go for it!” or “ABORT MISSION!”1

What follows is a preliminary overview of the role our nervous system plays in managing our emotional and, by extension, our behavioral states, and how this is affected by physical activities such as yoga and calisthenics. What we will consider is how your physiology is affected by a practice like yoga, and what actually changes in your body that makes this practice so effective in improving emotional and psychological states.

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The Vagus Nerve

Enter the Vagus Nerve. This nerve originates in the brain – specifically in the medulla oblongata, the control center for the heart and lungs – and terminates in the colon. It’s a giant, thick, cord that runs from your head, snakes through your chest, and ends in your butt.

“The aptly named vagus, latin for wandering, extends from brain to colon and modulates autonomic, cardiac, respiratory, gastrointestinal, immune and endocrine function. Like an unconscious brain, it integrates feelings from the body and regulates essential functions including heart rate, blood pressure and respiration.”3

This is the 10th cranial nerve and one among the thickest – nerve fibers, 80% of which are sensory, spread out from the vagus to various organs throughout the body, connecting and making up a large portion of the parasympathetic nervous system.3 Our current understanding of the vagus props it up as a massive eight-lane superhighway that transmits information to and from the brain and viscera. It’s a total feedback system that includes “motor pathways to change visceral state, sensory pathways to monitor visceral state and brain structures to evaluate the sensory input and to regulate the motor output.”3 Since the vagus‘ origin point is in the brain, it shares much of this information with other systems:

“Because the central regulator of the vagus in the brainstem is both an input and an output of other feedback systems, the vagal system becomes both a component of a more integrated neural feedback system and a portal to neural systems in other areas of the brain.”3

This is important to consider because it helps us understand the connection between exercise and the modulation of emotional states such as anxiety and depression: “stimulation of the vagal afferents regulate brain structures involved in epilepsy, depression, and even repetitive self-destructive behaviors often associated with autism.”3 Essentially, it is in stimulating the vagus that we find the benefits of physical exercise. But we want to know how it helps, to better understand what works and what doesn’t.

We’ll break it down a little bit, but return to more overarching themes that will help us understand the physiological impact of yoga a little bit better.

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Sympathetic and Parasympathetic Systems

Traditionally, the autonomic nervous system has been divided into two systems: sympathetic and parasympathetic. The sympathetic nervous system (SNS) is more commonly referred to as the “fight or flight” system, whereas the parasympathetic nervous system (PSNS) is more commonly referred to as the “rest and digest” system. The former controls adrenaline output, and is generally activated during stressful situations and times of duress. The latter manages activities within the body when the body is at rest, such as digestion and detoxification. Both systems are managed by the central nervous system, for which the vagus serves as the primary pathway for sensory communication between the brain and the periphery.

There is, however, a third system in play, and one that is slowly gaining traction in the fields of psychology and evolutionary biology (phylogeny). We’ll call this the Social Engagement System, and it is a vital part of Polyvagal Theory. This will prove beneficial in our discussion of the connection between our physiological, emotional, and behavioral states; Polyvagal Theory provides a framework by which we can understand this connection and how it works. This framework will help us understand why yoga is effective in dealing with emotional and psychological – as well as physical – ailments. The better part of the next section will deal predominantly with vagal tone and its effects on behavior, but the important take away is the physiological impact on the vagus that will serve in our discussion of yoga later on.

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Polyvagal Theory

Our most recent and currently widely accepted understanding of physiologically driven behavioral dysfunction offers four reasons as to why we may fall prey to various “dysfunctional mental states”: 1) excessive vagal outflow; 2) an imbalance between sympathetic and parasympathetic branches of the autonomic nervous system; 3) an excessive sympathetic outflow; and 4) a deficient vagal outflow.3 Now, I’ll not go into detail of each of these here; for our purpose here, it is sufficient that we understand that the current paradigm of dysfunctional behavior is due to an imbalance in the activation between the two primary nervous systems.3

Polyvagal theory introduces a new way of considering dysfunctional mental states and behavior, by which it “… provides a neurobiological model to explain how positive social behavior, social support and positive affective states might support health and growth.”3

Polyvagal Theory seeks to

“link between phylogenetic changes in the autonomic nervous system and social behavior. By focusing on the phylogenetic changes in the structure of the vagus and the role that vagus plays in the neural regulation of visceral state, new insights regarding social behavior emerge. Moreover, by articulating the phylogenetically organized hierarchy of neural circuits, insights into benefits of social behavior become evident as do an understanding of the behavioral and physiological features associated with stress and psychiatric disorders.”3

What this means is that by understanding how the vagus has evolved, we can better understand its role in social behavior. The theory suggests that the vagus has more to do with the “expression of behavioral, psychological and physiological features associated with social behavior”3 than previously understood. It is important to understand that our emotional and psychological well-being is closely tied to our physiology. Polyvagal Theory looks at our health and growth from a behavioral standpoint: external stimuli is processed and activates one of the two main systems in place: either the SNS or the PSNS. Given that we now know that “the central regulator of the vagus in the brainstem is both an input and an output of other feedback systems,” we may also look at our behavior from a health standpoint. Emerging research is connecting our physiological states with our behavior:

“Internal visceral states drive complex motivational behaviors both implicitly and through cognitive and affective responses, e.g., arousal, sickness behavior, memory, and mood. There is a primacy of interoceptive control to functional health; the fundamental experience and awareness of self is proposed to follow dynamic central representations of physiological state, driven by afferent visceral signaling.”6

The fundamental experience and awareness of self is proposed to follow dynamic central representations of physiological state. This connection is inherent in our speech patterns: “the fundamental association between bodily changes and emotions remains apparent in the universal use of physiological words to describe emotional states.”6 So, we attach physical associations to our emotional states and describe them as such. This means we intuitively understand the connection between our bodies and emotions. This serves as a conduit to better understand what is happening internally: the vagus, being central to the regulation of visceral states, plays a vital part in this connection. The more active the vagus, the greater control we have over our emotional states. How does this work?

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Vagal Tone and Behavior

Earlier, it was pointed out the vagus comprises a large portion of the PSNS. It is the level of activity within the vagus that determines how much or how little the PSNS is active in response to any stimuli. “Tone” is our ability to measure, to some extent, the activity within the vagus. The greater the vagal tone, the more potentially active the PSNS. Remember, PSNS refers to “rest and digest” as opposed to the SNS, which activates a state of higher alertness, or stress. Now, as mentioned before, the vagus originates in the medulla oblongata – the control center for the heart and lungs – and thus influences the operation of the heart and lungs along with other organs. Therefore, a more active vagus can result in a lower state of stress within the body:

Due to the tonic vagal influences on the sinoatrial node (i.e., the heart’s pacemaker), resting heart rate is substantially lower than the intrinsic rate of the cardiac pacemaker. When the vagal tone to the pacemaker is high, the vagus acts as a brake, slowing heart rate. When vagal tone to the pacemaker is low, there is little or no inhibition of the pacemaker and heart rate increases … In general, greater cardiac vagal tone produces slower heart rate and regulates the transitory changes in heart rate in response to stimulation.”3

A slower heart rate, as achieved by a more active vagus, can be indicative of the activation of the parasympathetic nervous system, turning the body’s attention towards “rest and digest,” indicating reduced levels of stress. So the heart rate slows down. We get a stronger parasympathetic response in our system. What does this mean?

“Functionally, the vagal brake, by modulating visceral state, enables the individual to rapidly engage and disengage with objects and other individuals and to promote self-soothing behaviors and calm behavioral states.”3

In essence, as parasympathetic response rises, we become better apt at managing stressful situations.

Like social functions.

And chair pose.

This activation is often automatic, though we can, to some extent, influence the activation of the PSNS by increasing vagal tone ourselves. This is accomplished by vagus nerve stimulation and is often used to treat epilepsy and major depression. “Vagal nerve stimulation is based on the assumption that stimulation of vagal afferents has a direct effect on the regulation of higher brain structures.”3 Returning to the fact that the central regulator of the vagus serves as both an input and output of other feedback systems, this makes sense: stimulating the vagus influences our physiology in such a way that our emotional response, and thus our behavior, is affected – “internal visceral states drive complex motivational behaviors both implicitly and through cognitive and affective responses.”6

Okay, cool. So how does this all apply to yoga?

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Putting it All Together

The primary method used to stimulate the vagus directly is a medical treatment that involves delivering electrical impulses to the vagus nerve. Currently, this method of treatment is particularly expensive and invasive; for most of us, this is not how we go about stimulating our vagus.

For most of us, we voluntarily stimulate our vagus with our breath. And since we began our discussion of the vagus in connection with yoga, here is where we connect the two: breathing exercises are a vital part of any yoga practice. As we do breath work in our yoga practice, we stimulate our vagus and thus begin to produce the physiological changes that occur in activating the PSNS. In extending this to other exercise modalities, it helps to consider the effort it takes to bring the breath under control after a strenuous workout: you are consciously activating the PSNS in this scenario.

This is not new information. Utilizing breathing practices such as pranayama to stimulate the vagus and increase vagal tone is well known to anyone who takes the time to seek out this information. However, most articles wax poetic about the wonders of the vagus and fail to explain how it’s conditioning works and why it is effective. Returning to my earlier statement regarding unsubstantiated claims, without understanding the physiological impact of a practice like yoga, it becomes easy to make exaggerated, and often bizarre, promises. Without taking the time ourselves to look into these claims, we perpetuate their spread.

What I have done here is establish a foundation of knowledge regarding the vagus nerve as it relates to PSNS activation, and it’s influence on visceral and behavioral states. It’s a starting point. In utilizing this knowledge, you can come to your own better-educated conclusions regarding some of the popular claims flying around in the world of yoga, and fitness in general. And where this knowledge falls short in evaluating a claim? Well, now you’re better equipped to do a little more digging.

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1Rawlings, J. (2015, January 2) Stretching is in Your Brain: A New Paradigm of Flexibility & Yoga (Part 1). Retrieved from

2Zeitlin, J. (2017, March 3) Yoga Needs to Cut the Bullshit. Retrieved from

3Pordges, S (2003, April 17) The Polyvagal Theory: phylogenetic contributions to social behavior. Retrieved from

4Streeter, C. C., Gerbarg, P. L., Whitfield, T. H., Owen, L., Johnston, J., Silveri, M. M., … Jensen, J. E. (2017, March 1) Treatment of Major Depressive Disorder with Iyengar Yoga and Coherent Breathing: a Randomized Controlled Dosing Study. Retrieved from

5Marangella, L. B., Rush, A. J., George, M. S., Sackeim, H. A., Johnson, C. R., Husain, M. M., Nahas, Z., Lisanby, S. H. (2002, February 15) Vagus nerve stimulation (VNS) for major depressive episodes: one year outcomes. Retrieved from

6Critchley, H. D., Harrison, N. A., (2013 February 20) Visceral Influences Brain and Behavior. Retrieved from

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